1. Field of the Invention
This invention relates to a large-aperture telephoto zoom lens applicable to an exchangeable lens for a silver halide film-type or digital single-lens reflex camera.
2. Description of Related Art
A number of telephoto zoom lenses have been proposed as exchangeable lenses for single-lens reflex cameras. Many of the telephoto zoom lenses are designed so that each zoom lens is constructed with four lens units having positive, negative, positive, and positive powers (refer to, for example, Japanese Patent Kokai No. 2000-19398, pp. 1-5, FIG. 1).
In most of telephoto zoom lenses such that a half angle of view ω is approximately 24° at a wide-angle position, that is, a focal length is about 100 mm in terms of a 35-mm film, and a variable magnification ratio is more than 3, an F-number is 4 or more at the wide-angle position. Furthermore, when an attempt is made to ensure brightness while increasing the focal length at a position closer to a telephoto position, a lens diameter and the entire lens length become oversized, and thus it is difficult to provide a telephoto zoom lens in which the variable magnification ratio is more than 3 and the F-number is small. In many of the telephoto zoom lenses, an object distance where it is short is nearly 1.5 m. A shorter object distance causes considerable fluctuation of spherical aberration, and it becomes difficult to increase the magnification of the zoom lens for photographing at a short distance.